fs/common: add option to execute custom scripts under fakeroot

fs/common: add option to execute custom scripts under fakeroot

Some users have the need to be able to tweak the content of the target
rootfs with root-like rights, that is, from inside the fakeroot script.

Add a new system option to allow those users to provide a list of
scripts, like the post-build and post-image scripts, that will be run
from our fakeroot script.

[Peter: pass TARGET_DIR to scripts, tweak help text]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Cam Hutchison <camh@xdna.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>